Absolutbestimmung der Gesamtanregungsquerschnitte der Edelgase durch Elektronenstoß

With an improved Maier-Leibnitz collision chamber absolute values of the total excitation cross sections of the rare gases were measured. The half width of energy distribution in the beam of exciting electrons was approximately 0.7 eV. The results for He and Ne are in reasonable agreement with the excitation functions given by Maier-Leibnitz, if some necessary corrections (especially with regard to contact potentials) are made. Only the cross sections obtained by us are a little smaller. Furthermore the better fine structure yielded more favourable possibilities of comparison with other measurements published so far. The error should not be greater than 30%.     

Linear-reactor-infrared-matrix and microwave spectroscopy of the cis-2-butene gas-phase ozonolysis

Investigation of the formation of complex products in the gas-phase ozonolysis of cis,-2-butene by linear-reactor-infrared-matrix and linear-reactor-microwave spectroscopy is reported. The following species have been unequivocally detected: secondary 2-butene ozonide, acetic acid, peracetic acid, glycolaldehyde, dimethyl ketene, the simple and mixed anhydrides of formic and acetic acid, 2,3-epoxybutane and 2-butanone, besides polyatomic products already known. In contrast, the primary ozonide has been detectable neither by LR.-MW. nor by LR.-IR. Observation of both stereoisomeric epoxides and kinetic modelling are used to support the intermediate formation of the O'Neal-Blumstein radical CH₃CH(O₂)CH(O)CH₃ and the existence of a reaction channel in which the two carbon atoms of the C, C double bond of the olefin remain connected. As the dominant reaction path a mechanism with a Criegee type split into methyldioxirane (ethylidene peroxide) and acetaldehyde is considered and subsequently proposed to explain formation of many complex products by either unimolecular or bimolecular processes of the peroxide. For the reactions considered, thermochemical estimates of reaction enthalpies and activation data are included. Kinetic modelling for a partial reaction mechanism involving at least two different paths of decay of the O'Neal-Blumstein biradical into Criegee-type intermediates and the 2, 3-epoxybutanes is discussed.     

Estimating the Sensory Irritating Potency of Airborne Nonreactive Volatile Organic Chemicals and Their Mixtures

Abstract

This article describes the possibility of estimating whether or not a mixture of nonreactive volatile organic chemicals (NRVOC) is likely to elicit complaints of sensory irritation in humans. For this estimation we rely on: a) the sensory irritating potency of individual NRVOC can be estimated from a variety of physicochemical properties of these chemicals, b) at low exposure concentrations, the additivity rule can be applied using the potency of each chemical in a mixture and c) a threshold concentration exists below which no sensory irritation will occur. We used this estimating approach and we compared the results obtained with those obtained experimentally in humans exposed to a well defined mixture. The approach presented can be used to arrive at a decision as to whether or not exposure to a mixture of NRVOC is likely to result in sensory irritation complaints by humans, either in the general indoor air situation or for industrial workers.     

Probing the antiknock effect of anisole through an ignition,speciation and modeling study of its blends with isooctane

In order to unravel the reaction pathways relevant to anisole co-oxidation within a fuel blend, a detailed study of isooctane/anisole blends was performed with the ULille RCM. Ignition delays as well as mole fraction profiles were measured during a two-stage ignition delay using sampling and GC techniques. These results are used to validate a kinetic model developed from ab initio calculations for the most relevant rate constants which included H-atom abstraction reactions from anisole, and reactions on the potential energy surfaces of methoxyphenyl + O₂ and anisyl + O₂. Pressure dependent rate constants were computed for the methoxyphenyl + O₂ and anisyl + O₂ reactive systems using master equation code analysis. The new kinetic model shows good agreement with the experimental data. Dual brute-force sensitivity analysis was performed, on both first- and second-stages of ignition, allowing the identification of the most important reactions in the prediction of both ignition delays. It was observed that while pure anisole does not show NTC behavior, a 60/40 isooctane/anisole blend displays such behavior, as well as two-stage ignition. This suggests anisole addition may not be as beneficial to knock resistance as expected from its high octane number. The kinetic modeling results demonstrate the importance of H-abstraction reactions both from the methoxy group and from the aryl ring in ortho-position and the addition of the resultant radicals to O₂, mostly leading to the formation of polar or non-aromatic products.     

Double Proton Tautomerism via Intra- or Intermolecular Pathways? The Case of Tetramethyl Reductic Acid Studied by Dynamic NMR: Hydrogen Bond Association,Solvent and Kinetic H/D Isotope Effects

Using dynamic liquid-state NMR spectroscopy a degenerate double proton tautomerism was detected in tetramethyl reductic acid (TMRA) dissolved in toluene-d₈ and in CD₂Cl₂. Similar to vitamin C, TMRA belongs to the class of reductones of biologically important compounds. The tautomerism involves an intramolecular HH transfer that interconverts the peripheric and the central positions of the two OH groups. It is slow in the NMR time scale around 200 K and fast at room temperature. Pseudo-first-order rate constants of the HH transfer and of the HD transfer after suitable deuteration were obtained by line shape analyses. Interestingly, the chemical shifts were found to be temperature dependent carrying information about an equilibrium between a hydrogen bonded dimer and a monomer forming two weak intramolecular hydrogen bonds. The structures of the monomer and the dimer are discussed. The latter may consist of several rapidly interconverting hydrogen-bonded associates. A way was found to obtain the enthalpies and entropies of dissociation, which allowed us to convert the pseudo-first-order rate constants of the reaction mixture into first-order rate constants of the tautomerization of the monomer. Surprisingly, these intrinsic rate constants were the same for toluene-d₈ and CD₂Cl₂, but in the latter solvent more monomer is formed. This finding is attributed to the dipole moment of the TMRA monomer, compensated in the dimer, and to the larger dielectric constant of CD₂Cl₂. Within the margin of error, the kinetic HH/HD isotope effects were found to be of the order of 3 but independent of temperature. That finding indicates a stepwise HH transfer involving a tunnel mechanism along a double barrier pathway. The Arrhenius curves were described in terms of the Bell–Limbach tunneling model.     

Binding of an acetic acid ligand to adenosine: a low-temperature NMR study

Binding of an acetic acid (HAc) ligand to adenosine (A) was studied by (1)H NMR spectroscopic techniques. Using a low-melting deuterated Freon mixture as solvent, liquid-state measurements could be performed in the slow exchange regime and allowed a detailed characterization of the formed associates. Thus, at 128 K, trimolecular complexes A.HAc(2) and A(2).HAc with both Watson-Crick and Hoogsteen sites of the central adenine base occupied coexist in various amounts depending on the adenosine:acetic acid molar ratio. Whereas the carboxylic acid OH proton is located closer to the acid for all hydrogen bonds formed, a more deshielded proton at the Watson-Crick site is evidence for a stronger hydrogen bond as compared to the Hoogsteen interaction. For the binding of acetic acid to an adenosine-thymidine base pair in either a Watson-Crick or a Hoogsteen configuration, hydrogen bonds to the available adenine binding site are strengthened as compared to the corresponding hydrogen bonds in the A.HAc(2) complex.